, Volume 17, Issue 5, pp 647-661

Activity Patterns of Harvester Ants Pogonomyrmex pronotalis and Pogonomyrmex rastratus in the Central Monte Desert, Argentina

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Abstract

Seed-eating ants could have a significant effect on plant communities in deserts and semiarid zones. This effect is mediated through spatial and temporal foraging patterns, and seed selection within patches. Foraging patterns of harvester ants in South American deserts are almost unknown. The purpose of this work is to determine the temporal variations in the activity levels of Pogonomyrmex pronotalis and P. rastratus in the central Monte desert, and how these patterns may be related to abiotic factors, particularly to soil temperature. Activity levels and soil surface temperature were recorded at hourly intervals in five colonies for each species during the activity season (October, December, February, and April) in both 1999–2000 and 2000–2001 periods (except for October 1999). Surface ant activity starts in October, increases between December and February, and then ceases by April. Surface ant activity is diurnal throughout the season and usually has a unique peak during midday in October and April, and two peaks in the morning and the afternoon from December to February. The proportion of the activity budget devoted to nest's maintenance activity was similar for both species. Activity levels of foraging workers tended to be higher in P. pronotalis than in P. rastratus. P. pronotalis is active between 20 and 59°C, with higher levels of activity between 35 and 45°C, whereas P. rastratus shows activity between 18 and 58°C, with higher levels of activity between 30 and 40°C. Our results suggest that temporal changes in surface activity respond mainly to soil temperature fluctuations. However, at intermediate temperatures (those probably encompassing the thermal tolerance range of these ant species), temperature appears not to be a good predictor of daily and seasonal activity fluctuations.